The invention relates to a device for producing an image on a printed image carrier comprising at least one writing unit designed in the form of an electrode for locally influencing a printing press printing image carrier in the form of halftone dots, said writing unit or units being adapted to be connected with a source of electrical power and to be operated in accordance with the image to be produced on the printing image carrier.
So-called erasable printing image carriers or plates have been proposed, which when mounted in the printing press may be used more than once, since the printing image produced on the carrier may be erased by heat, an electric current or the like so that the printing image carrier may then have a new image produced thereon by corresponding forms of energy.
The German patent No. 3,633,758 describes an example of such a system, in which the printing image carrier is made of a ferroelectric material and is able to be locally polarized to different extents so that the ability to be wetted by the printing ink is varied. For the operation of reversing polarity use may for instance be made of electrodes of which the one electrode is designed in the form of a stylus may be operated in accordance with the image to be transferred onto the printing image carrier, that is to say it may have an electric current applied to it for a short time. The second electrode has the printing image carrier mounted thereon.
A further system with erasable printing image carriers is described in German patent No. 3,705,439. In this case, the printing image carrier is made of an electrically conductive polymer which is able to be influenced electrochemically. In this case as well a careful design of the image transfer device is needed to produce electrical fields which are dot-like in cross section and have precise limits, for influencing the printing image carrier.
The problem occurring in the realization of such image transfer devices is that the switching over between current flowing and no current flowing and vice versa has to be carried out in very short time in order to comply with the requirements of higly sophisticated printing presses.